In recent years, with the rapid spread of information-related devices and communication devices such as personal computers, camcorders and cellular phones, it has become important to develop a battery for use as a power source for such devices. In the automobile industry, the development of high-power and high-capacity batteries for electric vehicles and hybrid vehicles has been promoted. Among various kinds of batteries, rechargeable lithium batteries have attracted attention due to their high energy density and high power.
Especially, rechargeable lithium-air batteries have attracted attention as a rechargeable lithium battery for electric vehicles and hybrid vehicles, which is required to have high energy density. Rechargeable lithium-air batteries use oxygen in the air as a cathode active material. Therefore, compared to conventional lithium rechargeable batteries containing a transition metal oxide (e.g., lithium cobaltate) as a cathode active material, rechargeable lithium-air batteries are able to have larger capacity.
In metal-air batteries, the cathode active material, oxygen, is not contained within the battery. Instead, this material is provided by the surrounding atmosphere. Naturally, such a system allows in principle a very high specific energy (energy provided by the battery per unit weight, typically given in Wh/kg in this technical field). In such batteries, oxygen may be partially reduced to peroxide, or fully reduced to hydroxide or oxide depending on the catalyst, electrolyte, availability of oxygen etc. When the negative electrode (anode) is lithium (Li), lithium peroxide (Li2O2) or lithium oxide (Li2O) may be formed.
A metal-air battery may be schematically represented in FIG. 1. It contains mainly the following parts:                metal anode (preferentially Li),        non-aqueous electrolyte,        air cathode (preferentially O2 cathode) most commonly and usually preferably based on carbon (but other cathode materials are known in this context), binder and sometimes catalyst.        
The ideal reactions during the use of such a battery should be as follows:
Upon discharge:At anode: Li→Li++e−At air cathode: 2Li++x/2O2+2e−→Li2Ox Upon charge:At anode: Li++e−→LiAt air cathode: Li2Ox→2Li++x/2O2+2e−
In the reaction which occurs in the air cathode upon discharge, the lithium ion (Li+) is dissolved from the anode by electrochemical oxidation and transferred to the air cathode through an electrolyte. The oxygen (O2) is supplied to the air cathode.
Nevertheless, during electrochemical processes of the battery, it can happen that the O2 or O2-derived species react with the solvent molecules of the electrolyte, which may lead to the formation of side reaction products such as Li2CO3, Li formate, Li acetate etc. These products are not desirable in the battery and are believed to reduce the metal-air battery performance.
These side-reactions may lead to poor re-chargeability of the system and poor capacity retention. These general problems may be illustrated schematically as shown in FIGS. 2 and 3.
The problems shown schematically in FIGS. 2 and 3 may be summarized as follows:                Problem 1: Low initial capacity. This is a problem for both primary and secondary metal-air non-aqueous batteries.        Problem 2: Low efficiency of system, characterized by a large voltage gap between charge and discharge voltages. This is only an issue for secondary metal-air non-aqueous batteries subjected to charging and discharging cycles.        Problem 3: Poor capacity retention, which leads to bad cyclability of the system and a low number of cycles because the capacity drops rapidly. This also is only an issue for secondary metal-air non-aqueous batteries.        Problem 4: The reaction process is slow and charge/discharge performances at high current are lower.        
A number of workers have investigated various electrolytes and gel polymer electrolytes mainly with a view to either improving the discharge capacities for primary applications (i.e. solve problem 1 as listed above), or to protect the metal (Li). Unlike most of the references cited below, the present invention addresses problems arising in rechargeable applications of metal-air batteries.    [Reference 1 (non-patent): J. Read, J. Electrochem. Soc. 149 (9), A1190-A1195, (2002)] describes the use of an electrolyte comprising LiPF6 in solvents selected from propylene carbonate (PC), γ-butyrolactone (γ-BL), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), and combinations thereof. Electrolytes with high O2 solubility correlated with high discharge capacity. PC:DEC was the preferred solvent combination.    [Reference 2 (non-patent): J. Read, J. Electrochem. Soc. 150 (10), A1351-A1356 (2003)] teaches that the discharge capacity is increased if electrolyte viscosity is decreased. It is taught that by increasing the O2 concentration and/or partial pressure, discharge capacities can be increased.    [Reference 3 (non-patent): J. Read, J. Electrochem. Soc. 153, (1), A96-A100, 2006] describes the use of ether-based electrolytes, such as 1,2-dimethoxyethane (DME), 1,3-dioxolane (DOL), with different salts. Rate capabilities are said to be improved by optimizing the electrolyte viscosity.    [Reference 4 (non-patent): W. Xu, J. Xiao, J. Zhang, D. Y. Wang, J. G. Zhang, J. Electrochem. Soc. 2009, 156, A773] teach that electrolyte polarity is a more important factor influencing the electrochemical performances than the oxygen solubility in the electrolyte. It is described that the effect of electrolyte viscosity and conductivity on performances is limited. The O2 diffusion rate through the open channels of the air electrode is several orders of magnitude higher than that through the liquid electrolyte. The open channels are strongly dependent on the polarity of solvent. Thus an electrolyte based on ethers & glymes can easily wet the surface of the carbon surface of the air electrode because these electrolytes have low polarity as well.    [Reference 5 (non-patent): W. Xu, J. Xiao, D. Wang, J. Zhuang, J-G. Zhang, JES, 157, (2), A219-A224, (2010)] teach that maximum capacity varies as a function of the amount of electrolyte. Also, the effect of TPFPB (tris(pentafluorophenyl)borane) as an additive in the electrolyte is studied. TPFPB facilitates the dissolution of large amounts of Li salts (such as LiF, Li2O and Li2O2) normally insoluble in organic solvents, but an increase in TPFPB % also leads to an increase of viscosity, which leads to a decrease of discharge capacity.    [Reference 6 (patent): US 2011/0059355 A1 (Battelle Memorial Institute)] describes how oxygen permeable membranes may be prepared to reduce overall battery weight and improve specific energy. The use of crown ethers as additive (e.g. 12-crown-4 or 15-crown-5) is described as increasing discharge capacity.    [Reference 7 (non-patent): W. Xu, J. Xiao, D. Wang, J. Zhang, J-G. Zhang, Electrochem. And Solid State Letters, 13, (4), A48-A51, (2010)] teaches the use of crown ethers used as additive with 1M lithium bis(trifluormethan-sulfonyl)imide (LiTFSI), propylene carbonate/ethylene carbonate (PC/EC) solvent. It is reported that when 15 wt % of 12-crown-4 or 15-crown-5 is added to the electrolyte, the capacity of Li/air cell increases by 28% and 16%, respectively.    [Reference 8 (non-patent): T. Kukobi, T. Okuyama, T. Ohsaki, N. Takami, J. Power Sources, 146, (2005), 766-769] describes a study of the influence of hydrophobic room temperature ionic liquids on discharge capacity.    [Reference 9 (non-patent): H. Ye, J. Xu, ECS Transactions, 3, (42), 73-81, (2008)] describes polymer electrolytes based on ionic liquids, such as salts of the TFSI anion (bis(trifluoromethanesulfonyl)imide), such as P13TFSI (P13 cation is 1-methyl-3-propylpyrrolidinium). The materials developed are taught to enable protection of Li, and reversible Li plating/stripping.    [Reference 10 (non-patent): D. Zhang, R. Li, T. Huang, A. Yu, Journal of Power Sources, 195, (2010), 1202-1206] discloses a composite polymer electrolyte for a Li-air battery. The principal goal is the protection of Li from water. A composite polymer electrolyte medium is prepared using a polymer component (PvDF-HFP), a lithium salt-ionic liquid (LiTFSI-PMMITFSI) and hydrophobic silica in a weight ratio of 27:70:3 wt %.    [Reference 11 (non-patent): J. Hassoun, F. Croce, M. Armand, B. Scosati, Angewandte Chemie Int. Ed. 2011, 50, 1-5] describe solid state ZrO2 added to PEO-LiCF3SO3 as solvent-free polymer electrolyte.    [Reference 12 (non-patent): Cormac O. Laoire, S. Mukerjee, E. J. Plitcha, M. A. Hendrickson, K. M. Abraham J. Phys. Chem. (2010)] describe a study of electrolytes TBAPF6 (tetrabutylammonium hexafluorophosphate) or LiPF6 in DMSO, DME, TEGDME, acetonitrile.    [Reference 13 (non-patent): Cormac O Laoire, S. Mukerjee, E. J. Plitcha, M. A. Hendrickson, K. M. Abraham J. Electrochem. Soc. 158 (3), A302-A308 (2011)] disclose a rechargeable Li-air cell using an electrolyte composed of LiPF6 in TEGDME (tetraethylene glycol dimethyl ether).    [Reference 14 (non-patent): S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé, P. G. Bruce Angew. Chem. Int. Ed., 50, 1-6, (2011)] describe a lithium-oxygen battery with ether-based electrolytes. It is described that Li2O2 is formed as major discharge product during discharge. However, Li2O2 is not the main discharged product any more after just five cycles.    [Reference 15 (patent): US2010/0266907 A1, R. Yazami] describes electrochemical systems with a solvent having metal-ions and oxygen dissolved therein, a fluorinated or metalloprotein oxygen dissolution enhancer provided in the solvent for enhancing dissolution of the oxygen in the solvent, a metal oxide dissolution enhancer, and a current collector in electrical contact with the solvent.    [Reference 16 (non-patent): S. S. Zhang, J. Read, J. Power Sources 196, 2011, 2867] discloses the use of tris(2,2,2-trifluoroethyl)phosphite (TTFP) (30% wt) as co-solvent in propylene carbonate (PC) to improve the discharge performances of Li-air batteries. However TTFP-based electrolytes might not be suitable for use in rechargeable Li-air batteries since the valence of the phosphorus is +3, and so it can potentially be oxidized to +5 to form tris(2,2,2-trifluoroethyl)phosphate TFP.    [Reference 17 (non-patent): S. S. Zhang, K. Xu, J. Read, J. Power Sources 196, 2011, 3906-3910] similarly teaches the use of LiCF3SO3 in PC/TFP as solvent blend with different concentrations.    [Reference 18 (patent): US2009/0239113 A1, Hase et al.] discloses systems in which the positive electrode and the non-aqueous electrolyte solution of the Li-air battery contain a compound having a stable radical skeleton (e.g. nitroxyl radical).
In the field of lithium-air batteries, various considerations have led to a generalized preference for carbonate or ether solvents. It has been required to offer a broad electrochemical stability window, i.e. a solvent that is stable over a wide potential range, and no noted instability with respect to anode or cathode components. Redeposition of lithium as dendrites has also been a problem that workers in the field have sought to avoid. Much work has focused on providing lithium salts that are soluble in the solvents used, and show good conductivity, and on how lithium salts may be protected.
The basic concept for designing organic electrolytes for lithium batteries are given below. These electrolytes are generally required to present some fundamental properties and most of the time are prepared so as to obtain a compromise of all the properties listed below:                High ionic conductivity        Thermal and chemical stability        Wide potential window (electrochemical stability)        Low reactivity toward other components in the battery (separator, current collectors, electrodes . . . )Additionally, it is best if those electrolytes are also:        Non-toxic        Safe, non-flammable        Inexpensive        
In the case of electrolytes for use in metal-air battery applications, a high O2 solubility and quick O2 diffusion could be considered as further desirable features.